The term actuator refers to a member of a class of mechanisms whose primary function is to provide a controllable force for moving an actuated element to a desired position. The actuator, the actuated element, any interconnecting linkage, and a power source comprise an actuator system. In addition to moving the actuated element, some actuator systems must also perform both a force limiting function to prevent damage should the system become jammed, and a position holding function to prevent forces generated by the actuated element from backdriving the actuator.
An aircraft flight control surface actuation system is an example of an actuation system requiring both a force limiting and position holding function. Without the force limiting function, the aircraft structure or the flight control surfaces themselves could be seriously damaged if the actuator continues to exert force after the system becomes jammed. Furthermore, the control surfaces are subjected to large aerodynamic loads during aircraft flight which can backdrive the actuation system, thereby allowing the control surfaces to shift from the desired position, absent some device in the actuation system for providing the position holding function. If the control surfaces shift from the desired position, the flight characteristics of the aircraft can be significantly impaired.
Actuators that convert rotary motion from a drive motor into linear motion of a reciprocating output member for moving an actuated element, such as an aircraft control surface, to a desired position often utilize recirculating ballscrews. Ballscrews provide large forces, due to the mechanical advantage of the screw, coupled with high power transmission efficiency gained through the low friction operation of the balls. This ability to generate high forces coupled with low friction operation generally require the addition of force limiting and position holding features where a ballscrew is utilized in an actuator of a highly loaded actuated element, such as a control surface of an aircraft.
Prior ball-screw type actuators have utilized a number of approaches to provide the necessary force limiting and position holding functions. Generally, however, prior ball-screw actuators utilized some form of a so-called "no-back" device located about the ball-screw shaft centerline. These no-backs have typically been relatively complex and bulky arrangements of friction plates, ratchet and pawl mechanisms, or ball-ramp driven torque limiting devices. U.S. Pat. No. 3,802,281 to Clarke is illustrative of this approach.
In other approaches, the drive motor is coupled to an input shaft of the actuator, with the input shaft being in turn coupled to drive the ball-screw via an irreversible gearset. Such a gearset can be provided by a crossed helical gear pair having a driven gear disposed about the centerline of the ballscrew and a drive gear attached to an input shaft oriented at a right angle to the ballscrew. If the drive gear is large enough in diameter in comparison to the driven gear, such a gearset is irreversible, i.e. the driven gear cannot backdrive the drive gear even if the ballscrew is axially loaded. Although such irreversible gearsets do provide the desired position holding function, the large drive gear diameter precludes their use in actuators which must be physically compact.
Prior actuators have sometimes also utilized torque limiting devices on their input shafts for providing the force limiting function. In such devices, the torque limiter locks the input against further rotation, or decouples the input when excessive axial force on the ballscrew causes the input torque to rise above a predetermined maximum valve. Experience has shown, however, that input torque limiters are prone to nuisance trips, due to their inability to compensate for inertia spikes or variations in drag torques, etc., that are inherently encountered during start-up of the actuator.
One prior ball-screw type actuator which provides a resettable force limiting function without resorting to the complex or bulky apparatus described above is illustrated by commonly assigned U.S. Pat. No. 4,459,867 to Jones. When an axial load on the ballscrew of Jones '867 exceeds a predetermined maximum value, belleville springs in the force limiter assembly of Jones' actuator allow the lead screw portion of the actuator to move axially and cause engagement of raked teeth on reaction plates. Full engagement of the teeth transfers force from the leadscrew to a housing of the actuator, thereby effectively locking the lead screw against further motion. By eliminating the ratchets, ball ramps, friction plates and input torque limiters of other prior actuators, Jones '867 provides an actuator which is more compact and reliable. Although the force limiter assembly of Jones '867 provides both a force limiting and a partial position holding function above the predetermined maximum force limit, after lock-up of the force limiter assembly, it provides neither of these functions below the maximum force limit and prior to lock-up of the force limiter.
In summary, most prior approaches to providing both a force limiting and a position holding function in a ball-screw type actuator have been too bulky for use in compact actuators. Input torque limiters have not been reliable. While the mechanism of Jones '867 provides a both force limiting and a partial position holding function above a maximum force limit more simply and in a smaller physical space than other prior ball-screw actuators, the force limiter of Jones provides neither function at axial loads below the maximum force limit. Such a force limiting mechanism by itself would thus not be capable, for instance, of maintaining the position of an aircraft control surface subjected to aerodynamic loads large enough to backdrive the actuator, but below the maximum force limit.
Therefore, it is an object of our invention to provide a compact actuator which will reliably provide both the required load limiting and position holding functions at all potential axial load levels which may be imposed on a reciprocating output member of the actuator. Further objects of our invention include providing:
1) an actuator as above, utilizing the load-limiting mechanism of Jones '867; PA1 2) an actuator as above which does not require an input torque limiter; PA1 3) an actuator as above which is suitable and adapted for use in a control surface actuation system for an aircraft; PA1 4) an actuator as above which can be manufactured at low cost; and PA1 5) a ballscrew type actuator as in 1-4 above.